Intermittent Fasting and Cortisol


Intermittent Fasting and Cortisol

This “article” started as a response to a Facebook thread I was tagged in yesterday where people were warning the original poster about exercising in the morning on an empty stomach due to it raising cortisol.

So let’s examine this very quickly.

Exercise also raises cortisol, so should we not exercise? Dieting will lead to more cortisol, as will low carbohydrate diets, coffee, etc. – do we warn people about that? We like cortisol during exercise as its release leads to mobilization of fatty acids from fat stores thus providing us with energy. After a workout where glycogen is depleted, cortisol is raised to spare glucose and then when you eat post-workout, it will offset this.

Cortisol is NOT bad – it’s just a hormone. It is released in a pulsatile fashion and its release in the morning is part of what helps us get moving. CHRONIC high levels of cortisol can certainly be a problem and negatively impact the immune system but acute responses are normal.  Please stop vilifying hormones like cortisol and insulin simply because you read a headline and don’t understand its functions in the body.

Transient increases during a short time of fasting or exercise or fasted exercise or a stressful situation, are not too much for our body to handle. If such a minor stressor (fasted exercise) was so detrimental, we would not have evolved to where we are today. As a matter of fact, one of the main benefits of intermittent fasting is its ability to boost the resiliency of cells in response to “stress.”

Cortisol and fasting are often pitted against each other in certain fitness circles. Those who preach, “Beware of cortisol increases in fasting,” are typically using cherry picked studies that don’t accurately depict the discussion at hand or simply ignore the entire picture.

For example, some of the common studies for those in this camp include one where the participants fasted for 5 straight days and cortisol increased. Well of course, why wouldn’t it? Your body is trying to spare glucose. This leads to clickbait-esque thinking…fasting = more cortisol and cortisol = bad therefore fasting = bad.

Next they might use some, not all (because they hurt their point), studies done on Ramadan. Here’s one thing to remember, they cannot drink water during their fast so they are severely dehydrated and they get one meal per day. And guess what? Dehydration also leads to increased cortisol – so is this an accurate way to judge fasting? Is your guru explaining all of this when they bash  fasting?

Those of us who use intermittent fasting would never recommend that. Once again, we cannot compare these scenarios to waking up and doing some cardio on an empty stomach, which has benefits in my opinion –

Fasted Cardio For Fat Loss


Here’s one, “testosterone/cortisol ratio values did not change significantly” Effects of Ramadan fasting on physical performance and metabolic, hormonal, and inflammatory parameters in middle-distance runners” (2009) —- These participants again with limited water intake and one meal per day for one month.

Here’s another, The proinflammatory cytokines IL-1β, IL-6, and tumor necrosis factor α; systolic and diastolic blood pressures; body weight; and body fat percentage were significantly lower (P < .05) during Ramadan as compared with before Ramadan or after the cessation of Ramadan fasting. These results indicate that RIF attenuates inflammatory status of the body by suppressing proinflammatory cytokine expression and decreasing body fat and circulating levels of leukocytes. (2012)

But are we really ONLY focused on one hormone? Does anything else matter?

What about – Alternate-day fasting trials of 3 to 12 weeks in duration appear to be effective at reducing body weight (≈3%–7%), body fat (≈3–5.5 kg), total cholesterol (≈10%–21%), and triglycerides (≈14%–42%) in normal-weight, overweight, and obese humans. Whole-day fasting trials lasting 12 to 24 weeks also reduce body weight (≈3%–9%) and body fat, and favorably improve blood lipids (≈5%–20% reduction in total cholesterol and ≈17%–50% reduction in triglycerides).” (2015)

Let’s use another Ramadan study done on well trained men, what did they find? Ramadan fasting was associated with a reduction of body mass and body fat. Lipolysis might have occurred because of increased plasma triglycerides and HDL cholesterol concentrations.” (2008)

These studies, if read closely, show pretty clearly that short-term fasts or fasting will not negatively effect average cortisol levels.

 My advice?

  • Stop relying on your facebook friends for information.
  • Stop vilifying single hormones, foods, exercises, etc. The human body is complex.
  • I will take improved blood glucose, lower LDL, higher HDL, lower body fat etc. for slight acute increases in cortisol.
  • Read my series on intermittent fasting and think for yourself . There are over 60 references.

    The Ultimate Guide To Intermittent Fasting I – Feast vs. Famine

2 Responses to “Intermittent Fasting and Cortisol”


  1.  Intermittent Fasting : Feast vs. Famine - Fred Duncan

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